Although it is generally accepted the arterial wall stiffens in response to sustained hypertension, the early response of the arterial wall to increased pressure has not been so clearly established. The objectives of this study are to study how the mechanical properties of the aorta change during externally induced hypertension in swine and then to determine using quantitative light and electron microscopic techniques the anatomic basis for any detected changes in the wall properties. To accomplish these goals, during the past year we measured the variation of aortic diameter and length with aortic pressure at weekly intervals for 1 month after the onset of deoxycorticosterone acetate (DOCA) induced hypertension in swine. Aortic diameter and length were measured using an ultrasonic strain gauge. The results of our study contrast with the results of some recent studies which show that the initial response of the arterial wall is to produce a more compliant blood vessel. We found no significant differences in the stress-strain relation between the group of hypertensive animals and a group of controls for a period of one month after implanting DOCA. The results of this study strongly suggest that structural adaptations of the arterial wall play no role in the initiation of the hypertensive process. Currently, we are studying how the structure of the aortic media changes during the first month of DOCA induced hypertension. Using quantitative light and electron microscopic techniques, we will determine what changes occur in the connective tissue matrix during this period of time. The results of this study will help us to understand the complex relationship between aortic structure and elasticity in the hypertensive animal.